1. Field of the Invention
The present invention relates to rotary steerable drilling systems, and more particularly to point-the-bit type systems.
2. The Related Art
An oil or gas well often has a subsurface section that is drilled directionally, i.e., inclined at an angle with respect to the vertical and with an inclination having a particular compass heading or azimuth. A typical procedure for drilling a directional wellbore is to remove the drill string and drill bit by which the initial, vertical section of the well was drilled using conventional rotary drilling techniques, and run in a mud motor having a bent housing at the lower end of the drill string which drives the bit in response to circulation of drilling fluid. A typical example of such a setup is Schlumberger's POWERPACK™ positive displacement steerable motor. The bent housing provides a bend angle such that the axis below the bend point, which corresponds to the rotation axis of the bit, has an inclination with respect to the vertical.
A “toolface” angle with respect to a reference, as viewed from above, is established by slowly rotating the drill string and observing the output of various orientation devices until the desired azimuth or compass heading is reached. The mud motor and drill bit are then lowered (i.e., the weight of the drill string is loaded onto the drill bit) with the drill string non-rotatable to maintain the selected toolface, and the drilling fluid pumps (a.k.a. “mud pumps”) are energized to develop fluid flow through the drill string and mud motor. The mud motor converts the hydraulic energy of the drilling fluid into rotary motion of a mud motor output shaft that drives the drill bit. The presence of the bend angle causes the bit to drill on a curve until a desired borehole inclination has been established. Once the desired inclination is are achieved at the desired azimuth, the drill string is then rotated so that its rotation is superimposed over that of the mud motor output shaft, which causes the bend section to merely orbit around the axis of the borehole so that the drill bit drills straight ahead at whatever inclination and azimuth have been established.
Various problems can arise when sections of the wellbore are being drilled with a mud motor and the drill string is not rotating. The reactive torque caused by operation of a mud motor can cause the toolface to gradually change so that the borehole is not being deepened at the desired azimuth. If not corrected, the wellbore may extend to a point that is too close to another wellbore, the wellbore may miss the desired subsurface target, or the wellbore may simply be of excessive length due to “wandering.” These undesirable factors can cause the drilling costs of the wellbore to be excessive and can decrease the drainage efficiency of fluid production from a subsurface formation of interest. Moreover, a non-rotating drill string will cause increased frictional drag so that there is less control over the “weight on bit” and the rate of drill bit penetration can decrease, which can also result in substantially increased drilling costs. Of course, a non-rotating drill string is also more likely to get stuck in the wellbore than a rotating one, particularly where the drill string extends through a permeable zone that causes significant build up of mud cake on the borehole wall.
Rotary steerable drilling systems minimize these risks by steering the drill string while it's being rotated. Rotary steerable systems, also known as “RSS,” may be generally classified as either “push-the-bit” systems or “point-the-bit” systems. The present invention is particularly related to the latter type of RSS.
In a point-the-bit rotary steerable systems, the axis of rotation of the drill bit is continuously deviated from the local axis of the bottom hole assembly (BHA) in the general direction of the next portion of the intended (curved) wellbore. Typically, the wellbore is propagated in accordance with a three-point geometry defined by upper and lower “touch points,” such as upper and lower stabilizers, and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and lower touch point results in the non-collinear condition required for a curve to be generated. In its idealized form, the drill bit is not required to cut sideways because the bit axis is continually rotated in the direction of the curved hole.
There are many ways in which the non-collinear condition may be achieved, including a fixed bend at a point in the BHA close to the lower touch point or a flexure of the drill bit drive shaft distributed between the upper and lower touch points. The continuous drill bit pointing required in a point-the-bit RSS can otherwise be achieved by non-rotating eccentric stabilizer sleeves. Eccentric sleeves rely on the angular orientation of the stabilizer relative to the wellbore to be constant to ensure the bit is pointed in the required direction. If relative rotation occurs between the stabilizer and the wellbore, the tool loses its reference point and needs to be re-set.
Still other systems, with all-rotating external components, maintain a fixed offset angle between the main tool axis and the bit axis through an offset mandrel and a universal joint. Those systems rely on high-power electric motors and control electronics to hold the drill bit axis in the desired orientation relative to the formation. When drilling straight, the control motor has to ensure the drill bit axis is rotating with the main tool to achieve a neutral steering response. The fixed offset angle requires a high-power motor to overcome the side frictional forces on the drill bit. A varying offset angle can reduce the power requirement but the additional degree-of-freedom needs to be controlled by some means.
A need therefore exists for a point-the-bit RSS where dogleg performance need not be compromised at the expense of maintaining steering direction.
A need further exists for a point-the-bit RSS that is not dependent on constant angular orientation between the wellbore and a component of the RSS.